Micro­power

  

Volume 2, Number 3 – July 1982











Page 10 of 37











clock. This means that a new access time slot must be decided upon, because which ever half of the 1 Mhz clock is used, it will always interfere with the load clock. The new access slot must be between load clock pulses, but not too close to the start of a load clock pulse so as to interfere with video access timing to the RAM. Figure 2 shows the new access signal, this is produced by exclusive ORing the 1 and 2 Mhz then inverting the resultant signal. The wait gate is produced as before and the phases of the 2 and 4 Mhz clocks altered to suit this using the theory previously described. Again all the waveforms are shown in figure 2 so that they may be compared to the original clocks etc.

Figure 3 shows the circuit diagram, this is fairly straight forward and should present few problems to the constructor. The final connection to the Nascom is somewhat more involved but, providing that the instructions are followed carefully, there should be no real problems

Construction.

In addition to the 4 integrated circuits used, IC 49 must be relocated on the Snowdinger 2 board. The socket for IC 49 must be of the wire-wrap type, with pins 2 and 6 cut short, This is so that the whole board can plug into Nascoms socket for IC 49, with the new 2 and 4 Mhz clocks connected, via wires, to pins 6 and 2 respectively. Any connections made to IC 49 should then be made to the wire wrap socket. The prototype used wire wrap sockets for all the connections but any method of construction may be used. Follow the circuit diagram carefully and check all your work as you go along.

Connection.

The board should have 6 wires for connection to the main board of the Nascom. In the circuit diagram of figure 3 these are marked 1 to 6. Using the following list make all your connections.

  1. Remove IC 58 completely from the main board.
  2. Remove IC 49 and place it on the Snowdinger 2, taking care to have it correctly orientated.
  3. Remove IC 71, bend pins 2 and 4 outwards and then replace it.
  4. Remove IC 69, bend pin 3 outwards and then replace it.
  5. Take wire 1 and push it into pin 2 of IC 49’s socket.
  6. Take wire 2 and push it into pin 6 of IC 49’s socket.
  7. Solder wire 3 to pin 3 of IC 69.
  8. Take wire 4 and push it into pin 9 of IC 58’s socket.
  9. Solder wire 5 to the processor end of R9 on the main board.

This is an OCR’d version of the scanned page and likely contains recognition errors.











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